Atari 2600, too. AtariAge.com runs a store that produces and sells cartridges of homebrew development efforts. http://atariage.com/store/index.php?l=product_list&c=21

If you think the Apple II is a challenge, try programming for the Atari 2600. 128 bytes RAM, 4K ROM, two hardware sprites. And no frame buffer; "video memory" consists of a couple dozen registers. The graphics chip is one-dimensional; to draw a picture you must refresh its registers after the electron beam draws each scanline. It literally uses the phosphor on the TV screen as the frame buffer.

You can do "racing the beam" tricks with midframe graphics changes on many machines, Apple II and C64 and Amiga and even a DOS PC, but it's required to even display a picture with the 2600.